1. Field of the Invention
The present invention relates generally to modulation-doped field-effect transistors and, more particularly, to fabrication processes for these transistors.
2. Description of the Related Art
Group III nitride compound semiconductors (e.g., gallium nitride, aluminum nitride, indium nitride and their alloys) have recently emerged as promising materials for high-power and high-temperature microwave performance. These nitride semiconductors have a higher chemical stability than previous semiconductors. In addition, they exhibit large bandgaps, high breakdown fields and high electron peak and saturated velocities. Accordingly, they have become the primary candidates for realizing high-power, high-temperature microwave amplifying devices (e.g., modulation-doped field-effect transistors (MODFET's)) in a variety of electronic fields (e.g., active array radars and wireless satellite communications).
However, some of the parameters of the group III nitride compound semiconductors which make their microwave performance attractive also present fabrication difficulties. For example, there does not exist at this time any wet chemical etchant that can controllably etch these compound semiconductors. In addition, the wide bandgap of these compound semiconductors causes them to be particularly susceptible to "charging" during electron-beam (e-beam) writing steps. Both e-beam lithography and optical photolithography techniques are widely used for defining and fabricating transistor details. However, the ability of optical photolithography to resolve transistor structures is limited, (e.g., to the region of one micrometer). Thus, optical photolithography techniques cannot generally produce the small gate lengths (e.g., on the order of 0.25 micrometer) which are desirable for microwave transistors.
Accordingly, e-beam lithography is typically used to define transistor structures of smaller size (e.g., gate lengths in the region of 0.25 micrometers). In contrast with optical photolithography, masks are not typically used in e-beam lithography to expose resist materials. Instead, a focused electron beam is rastered ("written") over the resist to expose it.
Although e-beams can write exceedingly small resist patterns, the accuracy of this process is degraded if the e-beam is deflected by spurious electric charges. This deflection is typically referred to as "charging" (e.g., see Williams, Ralph, Modern GaAs Processing Methods, Artech House, Norwood, Mass., 1990, pp. 140-146). It is accentuated by the presence of the high-resistivity substrates (e.g., sapphire and silicon carbide) which are preferably used with the group III nitride compound semiconductors. Charging is especially troublesome if the transistor has been isolated from surrounding structures because this fabrication step removes current paths which would otherwise be available for current discharge (e.g., see Williams, pp. 199-205).
In response to these fabrication problems, gates of group III nitride compound semiconductors have been typically fabricated with the use of optical photolithography. In addition, the lack of a chemical etchant has prevented selective etching to undercut gate leads and, thereby, reduce gate capacitance which degrades microwave cut-off frequencies. As a result, the microwave performance of transistors formed with group III nitride compound semiconductors has not realized the potential of these materials.